A heat pipe is a heat-transfer device that uses thermal conductivity and phase transition principles to effect the transfer of heat between two solid interfaces, e.g., a heat source (or hot interface) and a heat sink (or cold interface). Heat pipes are used in many applications to effect cooling. For example, heat pipes are one of the most effective means of electronics cooling on the market. They transfer energy primarily by latent heat which allows them to operate with minimal temperature drop between the heat source and the heat sink. Furthermore, they are passive devices, needing no input to operate or machinery to function.
A typical heat pipe includes a sealed casing made of a thermally conductive material (such as copper, aluminum or stainless steel) that is filled with a working fluid at a given pressure. One end of the heat pipe is at the heat source, and the other end is at the heat sink. To transfer heat between the heat source and the heat sink, the working fluid contacts the thermally conductive material of the casing at the heat source end (or hot interface), which when heated turns the working fluid into vapor. The vapor then travels to the heat sink end (or cold interface) where the vapor condenses back into a liquid and releases latent heat. Upon condensing back into the liquid phase, the working fluid travels back to the heat source end of the heat pipe, and the working fluid evaporates again, repeating the evaporation and condensation cycle.
Once the vapor created from the heat source condenses back to a liquid at the heat sink end of the heat pipe, the liquid must travel back to the heat source end to begin the cycle anew. To facilitate travel of the condensed liquid back to the heat source end of the heat pipe, gravity may be sufficient in certain circumstances. However, in some applications, the heat pipe may be oriented in a manner that does not allow gravity to facilitate movement of the liquid back to the heat source end of the heat pipe. For example, in certain applications, the heat pipe may be positioned such that the heat source end and the heat sink end are positioned horizontally rather than vertically. In these configurations, a wick may be used to facilitate movement of the liquid back to the heat source end by capillary action.
The working fluid conducts heat by a combination of sensible and latent heat. Desirable characteristics of the working fluid include high specific heat capacity, high surface tension, low contact angle, and low viscosity. High specific heat capacity enables large amounts of energy to be added to the liquid with little change in temperature, thereby minimizing the difference in temperature between the heater (or heat source) and condenser (or heat sink) required for operation. High surface tension helps provide ample capillary pressure to pump the liquid to the evaporating surface. A low contact angle provides a thin film evaporation region that is as large as possible, which allows for more heat transfer. Low viscosity reduces the amount of flow resistance required to move the liquid to the heat source to be evaporated.
Some typical working fluids include water, alcohols (such as methanol or ethanol), refrigerants (such as R134 (i.e., freon)), ammonia, and mixtures of water and alcohols. Selection of the material for the working fluid depends on the desired operating temperature and heat load. For example, for applications operating at extremely low temperatures, the working fluid might include liquid helium (2-4K), and for applications operating at extremely high temperatures, the working fluid might include mercury (523-923K), sodium (873-1473K), or indium (2000-3000K). For applications operating at low temperatures, some typical working fluids might include water (303-473K) or alcohols (e.g., methanol (283-403K) or ethanol (273-403K)). However, most heat pipes are offered between 233 and 473K, and use ammonia, alcohol, ethanol, water, or mixtures thereof.
Ammonia and alcohols do not have heat transfer effectiveness (i.e., small temperature differences to transfer large amounts of heat at moderate pressures) over a broad range of operating temperatures (e.g., about −25° C. to about 200° C.). Water can partly accomplish such heat transfer effectiveness over a broad range of operating temperatures, but can only be used in copper heat pipes. In particular, water has high specific heat capacity and a large enthalpy of vaporization, and may be used in many applications requiring a broad range of operating temperatures (i.e., about −25° C. to about 200° C.). Heat pipes that operate within this range are especially useful in electronic devices, regenerators in electrical power plants, aircraft carrier decks, and satellites. Accordingly, water is a particularly desirable working fluid for use in many applications. Indeed, water is frequently used in copper heat pipes. However, water cannot be used as the working fluid in aluminum heat pipes because a runaway chemical reaction takes place that forms non-condensable gasses (such as hydrogen gas), causing the heat pipe to fail. In particular, the formation of non-condensable gasses (NCGs) prevents energy from being rejected at the condenser.
Aluminum is light weight and low in cost, and has relatively high thermal conductivity (i.e., half that of copper). Aluminum is also approximately one third as dense as copper, and considerably cheaper than copper. As such, heat pipes made of aluminum are particularly desirable, and development of an aluminum heat pipe using water as the working fluid has been the subject of recent research. This research has generally focused on providing a coating on the inner surface of the aluminum pipe, which coating was intended to prevent the formation of NCGs. Some such attempts at devising an aluminum heat pipe using water as a working fluid included providing a coating having anti-corrosion properties on the internal surface of the aluminum pipe. However, these coatings are not replenished in the sealed heat pipe. As such, if the coating is compromised or damaged, NCGs will form, leading to rapid failure of the heat pipe.
In another attempt to reduce or prevent NCG formation in an aluminum heat pipe, an aqueous solution including specific inorganic salts is used as the working fluid. This solution creates a coating on the inner surface of the casing to protect the aluminum from corrosion and thereby prevent or reduce NCG formation. The continued presence of the inorganic salts in the working fluid allows the coating to self-heal if it is damaged or compromised. However, while this solution exhibits acceptable heat transfer properties and stable operation in aluminum heat transfer devices, the formulation includes toxic components (such as beryllium), and radioactive components (such as lawrencium). Other drawbacks to this solution include the propensity for particulate and sediment formation, and clogging of the wick. Moreover, heat pipes using this solution were manufactured in China under secret conditions by a single producer, and these heat pipes are believed to be no longer in production.